The present invention relates to expandable endoprosthesis devices, generally known as stents, which are designed for implantation in a patient""s body lumen, such as blood vessels, to maintain the patency thereof. These devices are particularly useful in the treatment and repair of blood vessels after a stenosis has been compressed by percutaneous transluminal coronary angioplasty (PTCA), percutaneous transluminal angioplasty (PTA), or removed by atherectomy or other means.
Stents are typically implanted within a vessel in a contracted state and expanded when in place in the vessel in order to maintain patency of the vessel to allow fluid flow through the vessel. Ideally, implantation of such stents is accomplished by mounting the stent on the balloon portion of a catheter, positioning the stent in a body lumen at the stenosis, and expanding the stent to an expanded state by inflation of the balloon within the stent. The stent can then be left in place by deflating the balloon and removing the catheter.
A bifurcated stenosis typically can occur in the carotid or coronary arteries at the carina between adjoining arterial branches and around the ostia of the adjoining arterial branches. Employment of a stent for repair of vessels that are diseased at a bifurcation requires that the stent must, without compromising blood flow, overlay the entire circumference of the ostium to a diseased portion and extend to a point within and beyond the diseased portion. Particularly at a bifurcation, lesions may form along the side walls of the blood vessel and at the carina of the bifurcation, not only contributing to stenosis of a main branch and side branch of the bifurcation, but also interfering with the normal rheology of flow at the bifurcation to create eddy currents that can contribute to formation of thrombosis.
A conventional stent might be placed so that a portion of the stent extends into the pathway of blood flow to a side branch of the bifurcation or extend so far as to completely cover the path of blood flow in a side branch. The conventional stent might alternatively be placed proximal to, but not entirely overlaying the circumference of the ostium to the diseased portion. Such placement of the conventional stent results in a bifurcation that is not completely repaired. Also, where the stent does not overlay the entire circumference of the ostium to the diseased portion, the stent fails to completely repair the bifurcated vessel.
In a conventional method for treating bifurcated vessels, the side-branch vessel is first stented so that the stent protrudes into the main vessel. A dilatation is then performed in the main vessel to open and stretch the stent struts extending across the lumen from the side-branch vessel. Thereafter, the main-vessel stent is implanted so that its proximal end overlaps with the side-branch vessel. However, the structure of the deployed stent must be recrossed with a wire by trial and error.
In another prior art procedure, known as xe2x80x9ckissingxe2x80x9d stents, a stent is implanted in the main vessel with a side-branch stent partially extending into the main vessel creating a double-barreled lumen of the two stents in the main vessel proximal to the bifurcation. Another prior art approach includes a so-called xe2x80x9ctrouser legs and seatxe2x80x9d approach, which includes implanting three stents, one stent in the side-branch vessel, a second stent in a distal portion of the main vessel, and a third stent, or a proximal stent, in the main vessel just proximal to the bifurcation.
In addition to problems encountered in treating disease involving bifurcations for vessel origins, difficulty is also encountered in treating disease confined to a vessel segment but extending very close to a distal branch point or bifurcation which is not diseased and does not require treatment. In such circumstances, very precise placement of a stent covering the distal segment, but not extending into the ostium of the distal side-branch, may be difficult or impossible.
It is important for stents to be sized correctly for the vessel into which they are implanted. In some situations, like the carotid artery, it is desirable to place a single stent from the common carotid artery to the internal carotid artery. The diameter is about 2 to 3 mm smaller in the internal carotid artery, so it is difficult to size a stent appropriately for both vessels. A stent that is designed for a large diameter vessel is not optimal for a small diameter vessel, and vice versa.
To address the deployment problems at a bifurcation and to address the stent sizing problems, the present invention is directed to a tapered stent. With such a tapered stent, the diameter of the stent varies along the length of the stent.
Some tapered stent designs are known in the art. For example, PCT Publication No. WO98/53759, entitled xe2x80x9cCarotid Stent,xe2x80x9d by Jay S. Yadav discloses a stent for cardiovascular application wherein a substantially cylindrical tubular member tapers from its proximal end to its distal end. This type of tapered stent is intended for stenting the common carotid bifurcation or the proximal internal carotid artery.
PCT Publication No. WO98/34668, entitled xe2x80x9cNon-Foreshortening Intraluminal Prosthesisxe2x80x9d by Gary S. Roubin et al. discloses an intraluminal prosthesis provided with a plurality of annular elements. The stent may be provided with varying flexibility along its length and/or circumference, and may include segments that have different diameters. The differing diameters may be accomplished by providing the stent in a tapered or a stepped configuration.
Other tapered stents include U.S. Pat. No. 5,222,964 to Cooper, disclosing a tapered stent made of resilient material for interconnecting portions of a Fallopian tube after a resection procedure. U.S. Pat. No. 5,180,392 to Skeie et al. discloses a prosthesis for use in joining hollow organ parts or systems wherein the prosthesis may have tapered outer ends. U.S. Pat. No. 4,441,215 to Kaster discloses a vascular graft of a synthetic material including a tubular member having a braided inner layer and a compliant outer covering layer. This synthetic vascular graft can have an increasing or decreasing taper.
Another tapered stent is known as the xe2x80x9cFlamingo Wallstent.xe2x80x9d The Flamingo Wallstent is intended for esophageal malignant strictures. It is partially covered at the ends to protect against tissue injury, and inside to prevent food impaction and tumor growth. A major drawback for the Flamingo Wallstent design is its inability to be accurately placed due to unpredictable foreshortening after deployment.
There is, however, still a need for an improved tapered stent for deployment in, for example, the common carotid bifurcation or the proximal internal carotid artery. These areas are the most common sites for cerebrovascular atherosclerotic disease.
To address the aforementioned problems, the present invention is directed to a stent having a taper along its length and having varying radial strength as a function of the diameter of the stent and spacing between the struts. In a preferred embodiment, the present invention is directed to a longitudinally flexible stent for implanting in a body lumen and expandable from a contracted condition to an expanded condition, comprising a plurality of adjacent cylindrical elements, each cylindrical element having a circumference extending around a longitudinal axis and being substantially independently expandable in the radial direction, wherein the plurality of adjacent cylindrical elements are arranged in alignment along the longitudinal stent axis, and wherein a plurality of cylindrical elements include sequentially increasing diameters to create a tapered profile, with each cylindrical element formed from struts arranged in a serpentine wave pattern; and a plurality of interconnecting members extending between the adjacent cylindrical elements and connecting the adjacent cylindrical elements to one another; wherein the struts and interconnecting members at the tapered profile increase in length along the longitudinal stent axis.
Such a tapered stent with smaller diameters as well as larger diameters has several benefits. A stent having a smaller diameter can have greater radial strength, better coverage of the vessel wall, and less foreshortening than is achievable with a stent having larger diameters. Obtaining these optimized features is especially important for the carotid application in which the internal carotid artery has the most significant disease, but the common carotid artery diameter dictates many of the design requirements of the stent.
As mentioned earlier, carotid stent procedures frequently involve the treatment of a diseased artery where plaque extends across the bifurcation between the common and internal carotid arteries. Selection of an appropriate stent diameter becomes precarious because the internal carotid artery tends to be smaller than the parent common carotid artery. The stent selected must be large enough to treat the common carotid artery, but using a stent sized to the common carotid artery can require implantation of a stent much larger than the nominal diameter of the internal carotid artery. This stent diameter mismatch and concomitant oversizing could lead to vessel injury and poor clinical results.
In the present invention, each end of the stent has preferably been designed specifically for the appropriate diameter range. That is, when deployed, the smaller diameter end of the stent supports the diseased portion of the internal carotid artery while the larger diameter end of the tapered stent supports the large diameter of the common carotid artery.
The present invention can be made from a shape-memory metallic alloy such as Nitinol or superelastic Nitinol to create a self-expanding stent. Alternatively, the present invention stent can be balloon expanded. With a balloon expandable stent, the shape of the balloon can be used to control the final shape of the stent. For example, a balloon with more than one diameter can be used to expand a stent having two final diameters. Separate balloons can also be used to post dilate the stent with a step in its diameter.
The present invention tapered stent presents a logical solution for carotid stenting across the bifurcation. The varying stent diameter accomplishes at least two goals: it allows adequate treatment of a lesion in both the common and internal carotid while maintaining a suitable stent-to-artery ratio for each vessel.